1. Field of the Invention
The invention is directed to transparent transportation of data signals with a continuous format, and in particular to a method for mapping arbitrary continuous signals into a SONET frame.
2. Background Art
Rapid provisioning of a customer's requested service is a valuable network function. There can be a large range of possible bit-rates for such services, or indeed the service and its bit-rate may not even have been defined when the network equipment is installed. Therefore, rapid provisioning of a service of arbitrary bit-rate is a valuable function.
Data transmission formats can be divided into SONET, other continuous formats, and burst formats. Burst formats do not have a continuous clock, transmission of such signals do not require any given phase relationship between bursts. On the other hand, the phase of the clock of continuous formats has continuity under normal conditions, and the frequency of the clock is bounded. Examples of such bounds are ±20 ppm (parts per million of the bit rate) and ±100 ppm.
The dominant signal format in the fiber optic networks follows the synchronous standard SONET in North America and SDH elsewhere. In this specification, SONET is defined to include SDH. SONET enables multiplexing, adding and dropping, and general transportation of signals. For a service, being able to be easily transported by a SONET network is a valuable attribute, in that it enables the network providers to make use of the large base of installed SONET-compatible equipment.
SONET is a physical carrier technology, which can provide a transport service for ATM, SMDS, frame relay, T1, E1, etc. As well, operation, administration, maintenance and provisioning (OAM&P) features of SONET provide the ability to reduce the amount of back-to-back multiplexing, and more importantly, network providers can reduce the operation cost of the network.
The SONET standards ANSI T1.105 and Bellcore GR-253-CORE, define the physical interface, optical line rates known as optical carrier (OC) signals, a frame format, and an OAM&P protocol. Opto/electrical conversion takes place at the periphery of the SONET network, where the optical signals are converted into a standard electrical format called the synchronous transport signal (STS), which is the equivalent of the optical signal. Namely, the STS signals are carried by a respective optical carrier, which is defined according to the STS that it carries. Thus, an STS-192 signal is carried by an OC-192 optical signal.
The STS-1 frame consists of 90 columns by 9 rows of bytes, the frame length is 125 microseconds. A frame comprises a transport overhead (TOH) occupying 3 columns by 9 rows of bytes, and a synchronous payload envelope (SPE) occupying 87 columns of 9 rows of bytes. The first column of the SPE is occupied by path overhead bytes.
As such, an STS-1 has a bit rate of 51.840 Mb/s. Lower rates are subsets of STS-1 and are known as virtual tributaries (VT), which may transport rates below DS3. Higher rates, STS-N, where N=1, 3, 12, . . . 192 or higher, are built by multiplexing tributaries of a lower rate, using SONET add/drop multiplexers. An STS-N signal is obtained by interleaving N STS-1 signals. For example, an STS-192 is made of 192 STS-1 tributaries, each separately visible, and separately aligned within the envelope. The individual tributaries could carry a different payload, each with a different destination.
The STS-N has a TOH made of all N TOHs of the individual tributaries, and a SPE made of all N SPEs of the tributaries, each with its own POH.
Some services, that operate at a higher rate, are transmitted in an STS-Nc signal (c for concatenation). The STS-1s into the STS-Nc signal are kept together. The whole envelope of the STS-Nc signal is routed, multiplexed and transported as a single entity rather than as N individual entities. The TOH and the start of the SPE for the N constituents are all aligned, since all the constituents are generated by the same source, with the same clock. The first STS-1 in the concatenated signal carries the single set of POH, all that is required for an STS-Nc.
Mapping of one rate or format into another is well known. Bellcore TR-0253 describes in detail the standard mappings of the common asynchronous transmission formats (DS0, DS1, DS2, DS3, etc) into SONET. Similar mappings are defined for the ETSI hierarchy mapping into SDH. Optical transmission equipment has mapped one proprietary format into another. For example, FD-565 could carry Nortel's FD-135 proprietary format as well as the DS3 standard format.
However, the standards or proprietary schemes allow transportation of a very specific set of signals, with format specific hardware. These methods of mapping cannot be used to map rates that vary significantly from the standard. Furthermore, these mappings are each precisely tuned for a particular format and a particular bit-rate, with e.g. a ±20 ppm tolerance. If a signal has, for example, a bit rate even 1% different than that of a DS3, cannot be transported within SONET. In addition, a different hardware unit is generally required to perform the mapping of each kind of signal.
A solution to the above problem is to add a “wrapper” to an arbitrary continuous signal. The rate of the resulting signal is a function of the signal being wrapped. Namely, a 1 MBps wrapper added to a signal of rate X produces a format with rate X+1 Mb/s. A variation on this adds a percentage of X. For example, a common line coding 8B/10 B produces a format with a rate of 125% of X. As such, the “wrapper” methods do not produce formats that have a pre-defined fixed bit rate for arbitrary inputs. The resulting signal cannot in general be time multiplexed to be transported on a high speed network.
U.S. Pat. No. 5,784,594 (Beatty) proposes a “TDM Wrapper” format where an arbitrary signal is mapped into as much of a frame as required, and the rest of the frame is left empty. However, this method requires a very large memory for each direction of conversion to hold the bits while waiting for the appropriate time slot to transmit them. As a result, this format is expensive to implement with high speed signals.
Packet or cell based formats map arbitrary input streams into SONET and SDH. While adequate for packet systems, these methods do not meet the jitter or wander requirements of most continuous signal formats due to the “one size fits all” mapping methods used. The clock phase information of the input signal is effectively eliminated in these methods, and so cannot be transmitted.
U.S. patent application Ser. No. 09/307812 (Solheim et al., entitled “Protocol Independent sub-rate device” filed on May 10, 1999 and assigned to Nortel Networks Corporation) discloses a method of transporting different type of clients (IP, ATM, SONET, Ethernet, etc.) together. The '812 application discloses time-multiplexing lower speed (subrate) channels of arbitrary rates and formats into a single higher speed channel, and then demultipexing the channels at the far end of the system. The portion of the bandwidth assigned to any given subrate channel can be provisioned without any change to the hardware or software. This significantly simplifies and speeds the provisioning of these services by the carrier. Tributaries with new protocols can be accommodated as well, significantly speeding up the delivery of support for these new protocols.
There remains a need for an efficient method and apparatus that will map arbitrary signals into SONET such that the signals can be recovered with low timing jitter at low cost.